1. Field of the Invention
This invention relates generally to active filter circuits and more particularly to an active filter circuit which is suitable for use with a semiconductor integrated circuit and the like.
2. Description of the Prior Art
FIGS. 1 and 2 show active filter circuits which have been developed for use with a semiconductor integrated circuit which is used to process a television signal and a video signal.
FIG. 1 shows one example of a conventional low pass filter. In FIG. 1, one side of an input signal source 1 is connected to the base of an npn-type transistor 2 and the other side of the input signal source 1 is grounded. The collector of transistor 2 is connected to a voltage source terminal 3 to which a positive DC voltage is applied. The emitter of the transistor 2 is connected to the emitter of an npn-type transistor 4, and the connection point between the emitters of transistors 2 and 4 is grounded via a constant current circuit 5 through which a predetermined current I flows. The collector of the transistor 4 is connected to the voltage source terminal 3 via a constant current source circuit 6 and the collector of this transistor 4 is grounded via a capacitor 7 which has capacitance C which forms a load. The collector of the transistor 4 is further connected to the base of an npn-type transistor 8 which forms an emitter-follower circuit of a feedback loop. The collector of the transistor 8 is connected to the voltage source terminal 3 and the emitter is grounded via a constant current circuit 9. The emitter of the transistor 8 is further connected to the base of the transistor 4 and an output terminal 10 is connected to the emitter of the transistor 8.
In principle, the active filter circuit shown in FIG. 1 forms a Gm (mutual conductance) feedback type primary filter circuit as shown in FIG. 3, and its filter characteristic H (.omega.) is given as ##EQU1## where re is the emitter resistance ##EQU2## of the transistors 2 and 4 and .omega. is the angular frequency.
The Gm feedback-type primary filter circuit shown in FIG. 3 will be described briefly. If the level of an input signal supplied to an input terminal 1a is L, the amplification factor of a differential amplifier circuit 2a is 1/r, the DC voltage supplied to a voltage source terminal 3a is H, the capacitance of a capacitor 7 is C and the output level generated at an output terminal 10 is X, then the following relations are established. ##EQU3##
FIG. 2 shows another example of the prior art primary low pass filter. The primary low pass filter of FIG. 2 employs a so-called Gilbert circuit.
Referring to FIG. 2, one side of the input signal source 1 is connected to the base of an npn-type transistor 11; and the other side is grounded. The emitter of the transistor 11 is grounded via a series circuit formed of a resistor 12 having a resistance value R and a constant current circuit 13 having a current I. The collector of the transistor 11 is connected to the emitter of an npn-type transistor 14 and the collector is connected to the voltage source terminal 3. The base of the transistor 14 is grounded via a battery 15. The collector of the transistor 11 is connected to the base of an npn-type transistor 16 and the collector is connected to the voltage source terminal 3 via a constant current circuit 17. The emitter of the transistor 16 is connected to the emitter of an npn-type transistor 18, and the connection point between the emitters of the transistors 16 and 18 is grounded via a constant current circuit 19 which has a constant current I. The collector of the transistor 18 is connected to the voltage source terminal 3. The base of the transistor 18 is connected to the collector of an npn-type transistor 20 and the emitter is connected through a resistor 21 having a resistance value R to the connection point between the resistor 12 and the constant current circuit 13. The collector of the transistor 20 is connected to the emitter of an npn-type transistor 22 and the collector is connected to the voltage source terminal 3. The base of the transistor 22 is connected to the base of the transistor 14. The collector of the transistor 16 is grounded via the capacitor 7 which has the capacitance C which forms a load. Also, the collector of the transistor 16 is connected to the base of an npn-type transistor 23 which forms an emitter-follower circuit of a feedback loop. The collector of the transistor 23 is connected to the voltage source terminal 3, and the emitter of the transistor 23 is grounded via a series circuit formed of a resistor 24 and a constant current circuit 25. The connection point between the resistor 24 and the constant current circuit 25 is connected to the base of the transistor 20 and an output terminal 10 is connected to the connection point between the resistor 24 and the constant current circuit 25.
In principle, the active filter circuit shown in FIG. 2 forms a Gm feedback type primary filter circuit such as shown in FIG. 3 and its filter characteristic H (.omega.) is given as ##EQU4##
The number of circuit elements constituting the active filter circuit shown in FIG. 1 is small and the dynamic range of this active filter circuit is Ire and the emitter resistance re of the transistor is very small, therefore the dynamic range of this active filter circuit is very narrow.
On the other hand, since the dynamic range of the active filter circuit shown in FIG. 2 is I (R+re), if the respective resistance values R of the resistors 12 and 21 are increased, the dynamic range can be increased. However, since such active filter circuits as shown in FIG. 2 require many circuit elements, when such active filter circuit is used for LSI (large-scaled integration) circuits which use a number of active filter circuits, the number of the circuit elements becomes very large.